As eyeglass flames, orthodontic elements, and biological replacement materials such as artificial bones, biocompatible and light titanium alloys have been employed. The biological replacement material desirably has elastic modulus (Young's modulus) of a low value close to that of the bone (about 30 GPa).
The present applicants has proposed a titanium alloy having a high corrosion resistance and also biocompatibility as a material for artificial bones and the like (Reference 1). This alloy is known under the name of “TNTZ alloy” and a representative alloy composition is Ti-29Nb-13Ta-4.6Zr. However, since the titanium alloy contains large amounts of Nb and Ta which are expensive materials, the alloy is unavoidably expensive as an alloy and also has a disadvantage that it is not easy to produce the alloy by melting since both Nb and Ta have high melting points (melting points of Nb and Ta are 2468° C. and 2996° C., respectively).
Subsequently, the applicant has proposed a Ti alloy having a composition comprising 20 to 60 weight % of Ta, 0.1 to 10 weight % of Zr, and the balance of Ti and inevitable impurities as a “substitute material for hard tissue” (Reference 2). The material exhibits a low Young's modulus in addition to biocompatibility and is suitable as a material for artificial joints and the like. However, since the titanium alloy contains a large amount of Ta which is an expensive material, the alloy is expensive as an alloy and also has the same disadvantage that it is not easy to produce the alloy by melting as in the case of the above TNTZ alloy since Ta has a high melting point as mentioned above.
Furthermore, the applicant has demonstrated an invention relating to “a biomedical Ti alloy and a process for producing the same” (Reference 3). The Ti alloy has an alloy composition comprising, by weight %, Nb: 25 to 35%; Ta in an amount so that Nb+0.8Ta is from 36 to 45%; Zr: 3 to 6%; O, N, and C in amounts so that O+1.6N+0.9C is 0.40% or less; and the balance of Ti and inevitable impurities. The merits of the Ti alloy are the points that it contains no components problematic in toxicity and allergenicity and has Young's modulus of 80 GPa or less but the disadvantage caused by the fact that it contains Ta in a high content still remain as in the case of the above substitute material for hard tissue.
Recently, there is disclosed a “titanium alloy” which has a low melting point and is easy to process, while it also has biocompatibility (Reference 4). This alloy is a beta-type titanium alloy comprising, by weight %, Nb: 25 to 35%; Zr: 5 to 20%; and at least one selected from Cr, Fe, and Si in an amount of 0.5% or more; and the balance of Ti and inevitable impurities. In this alloy, the use of Ti having a high melting point is avoided, and an alloy composition containing low-melting-point element(s) added is selected. However, the alloy still contains a large amount of Nb.
In addition, the production of the conventional titanium alloys uses pure metals as raw materials. Since there are a considerable number of high-melting-point components among the alloy components as mentioned above, production thereof by melting is carried out with difficulty and hence unavoidably costs high.    [Reference 1] JP-A-10-219375    [Reference 2] JP-A-2000-102602    [Reference 3] JP-A-2002-180168    [Reference 4] JP-A-2005-29845